Process for preparing metallocenes

Description

[0001] This invention relates generally to the preparation of metallocene derivatives which are useful as olefin polymerization catalysts and more specifically to an improved process for preparing bis(monosubstituted cyclopentadienyl) transition metal metallocenes which are substantially free of unsubstituted cyclopentadienyl impurities.

[0002] Metallocene derivatives represented by the formula (RCp)₂MX₂, where R is hydrocarbyl or silahydrocarbyl of 1 to 20 carbon atoms, Cp is cyclopentadienyl (C₅H₄-), M is titanium or zirconium and X is halogen are known to be useful catalysts for olefin polymerization. They can be prepared by reacting a deprotonated RCp compound with MX₄. As noted in the literature, however, the generation of RCp by the alkylation of cyclopentadiene using alkyl halides is difficult due to the generation of a complex mixture of products (Inorg. Chem. 1991, 30, 856-858). This reference proposes the use of either a liquid ammonia (low temperature) reaction medium or a highly reactive alkylating agent, such as ethyl trifluoromethane-sulfonate. Furthermore, the use of alkylhalides always results in a product which contains, due to the reaction equilibrium, cyclopentadiene and cyclopentadiene dimer. The dimer is inert in the subsequent metallocene preparation, but the cyclopentadiene must be removed. Otherwise, it will form (Cp)₂MX₂ impurities which adversely change the molecular weight distributions of the polymers produced when the (RCp)₂MX₂ metallocene product is used to catalyze olefin polymerization. Heretofore, complete removal of cyclopentadiene has not been achieved because of the generation of additional cyclopentadiene due to the cracking of the cyclopentadiene dimer. We have now discovered an economical, one-pot process for making such transition metal metallocenes which are free of Cp impurities. Furthermore, the process can be conducted at ambient temperatures which avoids the cost of refrigeration cooling systems.

[0003] In accordance with this invention there is provided a process for preparing a transition metal compound of the formula: (RC₅H₄)₂MX₂, where R is hydrocarbyl or silahydrocarbyl of 1 to 20 carbon atoms, M is titanium or zirconium and X is halogen, said process comprising the steps of:

(a) reacting Na(C₅H₅) with RX, where R and X are as defined above, in an organic solvent so as to form a reaction product mixture which includes RC₅H₄ and C₅H₅;

(b) vacuum stripping said product mixture at ambient temperatures so as to remove substantially all of said C₅H₅ from said product mixture;

(c) deprotonating said RC₅H₄; and

(d) adding MX₄, where M and X are as defined above, to said product mixture so as to react said deprotonated RC₅H₄ and said MX₄ and form said transition metal compound, said compound being substantially free of C₅H₅ containing impurities.

[0004] According to the alkylation process, NaCp is reacted with an alkyl halide in a solvent. The NaCp is commercially available and can be prepared by cracking cyclopentadiene dimer and reacting the cyclopentadiene with sodium metal as is known in the art. The preferred alkyl halides are represented by the formula RX, where R is C₁ to C₂₀ (and more preferably C₄ to C₁₀) hydrocarbyl or silahydrocarbyl such as, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, isobutyl, pentyl, hexyl, heptyl, or trimethylsilyl and X is halogen. Bromine is the preferred halogen. Chlorine can be used but gives slower reaction rates and iodine is more expensive. The NaCp and RX reactants are preferably used in NaCp:RX mole ratios of about 1:1, in order to minimize the amount of dialkyl substituted or unsubstituted Cp and Cp₂ (cyclopentadiene dimer) by-products, but ratios of 1:0.8 to 1:1.2 can be used. The preferred solvents for the reaction are ethers (e.g., THF or diethylether but hydrocarbon solvents can also be used such as benzene or toluene. Preferably, combined reactant concentrations of from 5 to 20 wt. percent in the solvent, are used. The reaction can be conducted at ambient temperatures with the reaction being exothermic. Generally, the reaction temperature will range from 20° to 50° C. The by-products of the alkylation reaction typically include besides NaCl, minor portions of R₂Cp and Cp/(Cp₂). Surprisingly, we have found that the Cp can be substantially completely removed (less than about 1 wt. percent) by reducing the volume of the reaction mixture under vacuum (2-15 mm Hg) at temperatures of 0° to 25° C. The cyclopentadiene dimer remains, but is inert so long as the subsequent reactions and product work-up procedures avoid elevated temperatures which would cause cracking to form additional cyclopentadiene. The metallocene by-products resulting from the presence of some disubstituted cyclopentadiene (R₂Cp) can be easily separated from the final metallocene product by solvent separation. The vacuum treatment also removes any unreacted alkyl halide which can then be recycled. Reduction of the volume of the reaction mixture to about 25% is sufficient to remove substantially all of the cyclopentadiene (no Cp was detected by GC such that the Cp content was less than about 0.1%.)

[0005] The product RCp can then be deprotonated in situ with, for example, Na powder, BuLi, NaH, LiH or a Grignard reagent (RMgX). Solvent can be added back to the reaction mixture prior to deprotonation. Preferably, about a 1:1 mole ratio of RCp to deprotonating agent is used. No more than a 10% excess of deprotonating agent is necessary or desirable as we have found that salt formation with the mono-alkyl product is favored over the dialkyl (R₂Cp) impurity. The deprotonation can be carried out at ambient temperatures and cooling is unnecessary, although higher or lower temperatures of -30° C to 60° C can also be used. If sodium is used as the deprotonating agent, then excess sodium metal must be filtered from the reaction mixture prior to the further reaction with the transition metal halide to form the product bis(mono-alkylcyclopentadienyl) transition metal metallocene.

[0006] The (RCp)₂MX₂ metallocene derivatives are prepared by adding a transition metal halide of titanium or zirconium, e.g., TiCl₄ or ZrCl₄, to the deprotonated mono-alkylsubstituted cyclo-pentadiene ligand, preferably in nearly stoichiometric proportions of ligand to transition metal halide (mole ratios of 1:0.4 to 0.5 ligand to transition metal halide and, more preferably, 1.0:0.5) so as to minimize the formation of RCpMX₃ impurities and the amount of unreacted transition metal halide which may be difficult to remove from the product. The process can be carried out at ambient temperature. Generally, the reaction temperatures range from 20° to 40° C.

[0007] When using sodium as the deprotonating agent, it is preferable to add the transition metal halide as a solvent solution in an ether solvent such as glyme, diethylether or tetrahydrofuran (TNF). The sodium halide which is formed will precipitate and can be filtered from the product solution. When BuLi is used as the deprotonating agent, LiCl is soluble in tetrahydrofuran but not in diethylether so that the preferred reaction solvent is diethylether. As an alternative when using BuLi, the alkyl cyclopentadiene reactant can be dissolved in diethylether and the BuLi in hexane so as to form a slurry of LiRCp. The transition metal halide such as ZrCl₄ powder or TiCl₄ can then be added. LiCl will precipitate and can be removed by filtration. Upon stripping the ether, the product (monoalkylCp)₂ZrCl₂ product will crystallize from the hexane. An advantage of the process of the invention is that it can be carried out as a one-pot process without the need to recover any intermediates. The products can be recrystallized by dissolving them in warm hexane (50°C) and then cooling the hexane solution to precipitate the purified products.

[0008] The process is further illustrated by, but is not intended to be limited to, the following examples.

Example 1

[0009] BuBr (12 mmol, 1.644 g) and THF (6.0 g, 6.8 ml) were charged to a 50 ml r.b. flask in a dry box. A mixture of NaCp (10 mmol, 0.88 g) and THF (14 ml) was added slowly at 22 to 40°C into above BuBr/THF solution over a period of 15 minutes. The reaction mixture was stirred at 22°C for 3 hrs. to form a mix-ture of BuCp (58 mol %), Bu₂Cp (21 mol %), Cp/(Cp)₂ (21 mol %) based on NaCp charged, and a trace of BuBr. The volume of the above mixture was reduced under vacuum (0° to 22°C/2mm Hg) to 25% of its original volume. GC showed that all the cyclopentadiene and BuBr had been removed and there was 40 mol % of BuCp based on NaCp charged in the product mixture. About 20 ml of THF were then added. Deprotonation with a 10% excess amount of Na powder was performed. The Na was slowly added into above solution at 22°C over a period of 1/2 hr. Hydrogen formation was observed. The mixture was then stirred at 22°C for an additional 2 hrs. ¹HNMR of the product showed a > 95% yield of NaBuCP. After the Na and NaBr were filtered out, the NaBuCp was further reacted with ZrCl₄ (0.46 g, 2 mmol, dissolved in 4 g. of glyme) at 22°C which was slowly added into the THF solution of NaBuCp in 5 minutes to form a > 90% yield (by ¹HNMR) of (BuCp)₂ZrCl₂ which contained no cyclopentadienyl zirconocenes. The glyme and THF were removed and an equal volume of Et₂O was added in order to precipitate LiCl, which was then filtered out. Hexane (15 ml) was added, and the Et₂O was slowly stripped off at 40 to 45°C. The hexane solution was cooled to 22°C and (BuCp)₂ZrCl₂ crystallized. The product was filtered out, dried, and weighed. The yield was 56% based on the BuCp used. The ¹HNMR showed that this product contained 6% (Bu₂Cp)₂ZrCl₂ but no cyclopentadienyl zirconocenes. MP 89-92°C (lit 93°C). Pure product can be obtained by further recrystallization.

Example 2 (Comparison)

[0010] BuCp (94% pure, 6.5 g, 0.05 mole) which was not prepared by the process of Example 1, but was purified by distillation and still contained 6% cyclopentadiene, and 200 ml 2of Et₂O were charged to a 500 ml flask in a dry-box. BuLi (0.05 mole, 3.2 g) in 20 ml of hexanes was added slowly with stirring into the above solution (temperature 22° to 33°C over a period of 30 minutes to form a very thick LiBuCp/Et₂O slurry. The reaction mixture was stirred at 22°C for an additional 30 minutes. ¹HNMR showed a 95% conversion. ZrCl₄ powder (0.025 mol) was directly added to the above slurry with stirring (temperature 20° to 33°C over a period of 30 minutes. The reaction mixture was stirred at 22°C for an additional 30 minutes. ¹HNMR showed a 95% formation of (BuCp)₂ZrCl₂. In the Et₂O/hexane solvent reaction medium, the LiCl was 100% precipitated. The LiCl was filtered out, 40 ml of hexane were added to the reaction mixture and the Et₂O was stripped off at 40-45°C. The hexane solution was cooled to 22°C and (BuCp)₂ZrCl₂ crystallized. The product (BuCp)₂ZrCl₂ was filtered out, dried, and weighed. The yield was 74%. An additional 9% (BuCp)₂ZrCl₂ was recovered from the mother liquor. The product also contained 5% cyclopentadienyl zirconocenes. The example illustrates that the Cp impurity present in the starting BuCp will be carried through into the product metallocene. The example also illustrates an alternate method of deprotonation and product formation which can be used in the process of the invention.

Claims

1. A process for preparing a transition metal compound of the formula: (RC₅H₄)₂MX₂, where R is hydrocarbyl or silahydrocarbyl of 1 to 20 carbon atoms, M is titanium or zirconium and X is halogen, said process comprising the steps of:

(a) reacting Na(C₅H₅) with RX, where R and X are as defined above, in an organic solvent so as to form a reaction product mixture which includes RC₅H₄ and C₅H₅;

(b) vacuum stripping said product mixture at ambient temperatures so as to remove substantially all of said C₅H₅ from said product mixture;

(c) deprotonating said RC₅H₄ and

(d) adding MX₄, where M and X are as defined above, to said product mixture so as to react said deprotonated RC₅H₄ and said MX₄ and form said transition metal compound, said compound being substantially free of C₅H₅ containing impurities.

2. The process of claim 1 wherein said RC₅H₄ is deprotonated without recovering said RC₅H₄ from the reaction mixture.

3. The process of claim 1 wherein R is C₄ to C₁₀ hydrocarbyl and X is chlorine.

4. The process of claim 1 wherein the compound is bis(nbutylcyclopentadienyl) zirconium dichloride.

5. The process of claim 1 wherein the mole ratio of Na(C₅H₅) to RX is from 1:0.8 to 1:1.2 and the mole ratio of deprotonated RC₅H₄ to MX₄ is from 1:0.4 to 1:0.6.

6. The process of claim 5 wherein said product mixture is vacuum stripped at a pressure of from 2-15 mm Hg and temperatures of 0 to 25°C.

7. The process of claim 6 wherein said organic solvent is selected from the group consisting of ethers, hydrocarbons and any combination thereof.

8. The process of claim 7 wherein said solvent is tetrahydrofuran, RX is n-butylbromide, MX₄ is ZrCl₄, the transition metal compound is bis-(n-butylcyclopentadienyl) zirconium dichloride, and said RC₅H₄ is deprotonated with a deprotonating agent selected from the group consisting of Na, NaH, LiH, BuLi and a Grignard reagent.

9. The process of claim 8 wherein said deprotonating agent is Na and said ZrCl₄ is added as a solution in an ether solvent.

10. The process of claim 8 wherein said deprotonating agent is BuLi in hexane, said organic solvent is diethylether such that said BuLi and said R(C₅H₄) form a slurry containing LiBuC₅H₄ and said ZrCl₄ is added as a solid powder to said slurry to form said bis-(n-butylcyclopentadienyl) zirconium chloride and LiCl, which LiCl precipitates and is removed from the product solution.